people = (('贾兴科', 'China'),
          ('Tom', 'England'),
          ('Bob', 'Nigeria'),
          ('王思聪', 'China'),
          ('Nagata', 'Japan'),
          ('Suzuki', 'Japan'),
          ('Hou', 'Taiwan')
          )
print(people)

countries = set()
for i in people:
    name, country = i
    countries.add(country)

print(countries)

countries.remove('China')
print(countries)
# countries.remove('China')
# print(countries)

countries.discard('China')
print(countries)

c = countries.pop()
print(c, countries)

countries.clear()
print(countries)
